1. Field of the Invention
This invention relates to a constant current LED lamp with a linear driver.
2. Description of the Related Art
Light-emitting diodes (LEDs) for general illumination applications are gaining popularity for their long service life and low power consumption. In such applications, the LEDs have to be provided with a driver circuit to convert the AC power of the mains to DC, for the LEDs are driven by DC current.
As shown in FIG. 1(A), an LED driver circuit 1 is connected to at least one LED 12 and adapted to receive an AC power VAC. The LED driver circuit 1 is a switching mode power supply, which mainly comprises a bridge rectifier 10, an electromagnetic interference restraining circuit 11, a filter circuit 16, a high frequency switching circuit 13, a converter circuit 14, and a feedback circuit 15, wherein the bridge rectifier 10 is used to rectify the VAC into a pulsed DC power, while the filter circuit 16 dampens the voltage swings of the DC power. The frequency of the DC power is controlled by the high frequency switching circuit 13, and the converter circuit 14 is used to convert the high voltage into a low voltage DC power that is supplied to the LED 12. The feedback circuit 15 can detect the magnitude of total voltage load of the LED 12, and thereby regulate the output voltage of the converter circuit 14.
Although the conventional LED driver circuit 1 does drive the LED 12 to emit light, the architecture of the driver circuit is quite complicated, which results in a large circuit and a high cost. When the cost becomes a concern, the electromagnetic interference restraining circuit 11 is usually omitted, or the component specifications are downgraded, which could lead to poor power conversion efficiency.
A second conventional LED driver circuit is shown in FIG. 1(B). The LED driver circuit 1 mainly comprises a capacitor C and a bridge rectifier 10, the capacitance of capacitor C determines the voltage supplied to the rectifier, whereas the bridge rectifier 10 rectifies the AC power to a pulsed DC power that is supplied to the LED 12.
Although the second conventional LED driver circuit 1 does drive the LED 12 to emit light, this circuit is more suitable for low power applications. Since the capacitor C typically is a high voltage plastic capacitor of small capacitance and large size, in the case of higher power applications, several capacitors have to be connected in parallel to increase the capacitance, which leads to increased circuit size and cost.